Back to EveryPatent.com
| United States Patent |
6,125,394
|
|
Rabinovich
|
September 26, 2000
|
Computer system having a plurality of resources and utilizing a
selection mechanism to select the resources based upon historical
loading
Abstract
A computer system in which resources are selected or purged based on
extremes of utilization (i.e., by virtue of having either the highest or
lowest utilization ranking in a group of resources) effectively handles
newly added resources whose initial utilization rankings do not reflect
their true popularity (or lack thereof). New resources are arbitrarily
assigned an initial utilization ranking that is in the middle of the range
of utilization rankings--e.g., preferably the median or, more preferably,
the mean--of other members of the group. Therefore, the new resource is
not immediately either selected or purged, because its initial utilization
ranking is not at either extreme, and eventually, after not too long an
interval, the utilization ranking reflects the true conditions, and the
resource will be selected or purged, or not, as the statistics may
warrant. In the meantime, other resources in the group are selected or
purged based on their actual statistics which reflect the true conditions
closely enough.
| Inventors:
|
Rabinovich; Michael (Gillette, NJ)
|
| Assignee:
|
AT&T Corporation (New York, NY)
|
| Appl. No.:
|
869530 |
| Filed:
|
June 6, 1997 |
| Current U.S. Class: |
709/226; 709/220; 709/225; 709/229; 711/133; 711/134; 714/38; 717/127; 718/100; 718/104; 718/105 |
| Intern'l Class: |
G06F 013/00; G06F 013/42; G06F 015/163 |
| Field of Search: |
395/672,673,675,676,704,183.14
711/133,134
709/100,105,104,220,226,225,229
|
References Cited
U.S. Patent Documents
| 5301323 | Apr., 1994 | Maeurer et al. | 395/650.
|
| 5603029 | Feb., 1997 | Aman et al. | 395/675.
|
| 5692193 | Nov., 1997 | Jagannathan et al. | 709/106.
|
| 5745703 | Apr., 1998 | Cejtin et al. | 709/238.
|
| 5745778 | Apr., 1998 | Alfieri | 395/800.
|
Other References
Keith Lopez, MACH 3 Kernel Principles, Open Software Foundation and Carngie
Mellon University, Jul. 15, 1992.
David L. Black, The Mach cpuserver: An Implementation of Processor
Allocation, Aug. 14, 1990.
|
Primary Examiner: Rinehart; Mark H.
Assistant Examiner: Vaughn, Jr.; William C.
Attorney, Agent or Firm: Kenyon & Kenyon
Claims
What is claimed is:
1. A computer system comprising;
a plurality of resources of different types, at least two of the plurality
of resources being in a group of resources of a particular type from which
a resource is selected, each resource of the plurality being connected for
communication with and utilization by at least one resource of the
plurality; and
a selection unit for,
i) when a resource from the group having resources of a particular type is
required to be selected, selecting from among said resources of the group
based on historical data regarding utilization of said resources of the
group, and
ii) determining whether a resource of the group should be eliminated from
the group when a new resource is added to the group, and if so,
determining which resource of the group should be eliminated from the
group when the new resource is added to the group,
said selection unit comprising:
a statistical unit for maintaining utilization rankings for said resources
of the group based on historical data regarding utilization of said
resources of the group, wherein, for a particular subset of said resources
of the group comprising a plurality of members of said particular type,
one of said members has a minimum utilization ranking and another of said
members has a maximum utilization ranking, said statistical unit
identifying said one of said members having one of (a) said minimum
utilization ranking, and (b) said maximum utilization ranking, and
a new resource initialization unit for assigning to a resource that is
newly added to said group, said newly added resource being of said
particular type and not initially in said group, an initial utilization
ranking between said minimum utilization ranking for said one of said
particular type and said maximum utilization ranking for said one of said
particular type.
2. The computer system of claim 1 wherein, when a particular resource of
one of said types is utilized, said statistical unit updates at least said
utilization ranking for said particular resource of said one of said
types.
3. The computer system of claim 1 wherein said new resource initialization
unit assigns to said newly available resource an initial utilization
ranking that is a mean of said utilization rankings for said one of said
types.
4. The computer system of claim 1 wherein said new resource initialization
unit assigns to said newly available resource an initial utilization
ranking that is a median of said utilization rankings for said one of said
types.
5. The computer system of claim 1 wherein said one of said types is cached
data objects.
6. The computer system of claim 1 wherein said one of said types is
processors.
7. A computer system comprising:
a plurality of processors, each of said processors capable of performing
tasks to be carried out by said computer system; and
a selection unit for,
i) when a particular task is to be carried out, selecting a processor from
among a group of at least some of said processors based on historical data
regarding utilization of said processors of the group, and
ii) determining whether a processor of the group should be eliminated from
the group when a new processor is added to the group, and if so,
determining which processor of the group should be eliminated from the
group when the new resource is added to the group,
said selection unit comprising:
a statistical unit for maintaining utilization rankings for said processors
in said group based on historical data regarding utilization of said
processors in said group, one of said processors in said group having a
minimum utilization ranking and another of said processors having a
maximum utilization ranking, said statistical unit identifying said one of
said processors in said group having said minimum utilization ranking, and
a new processor initialization unit for assigning to a processor that is
newly added to said group, said newly added processor not initially in
said group, an initial utilization ranking between said minimum
utilization ranking and said maximum utilization ranking.
8. The computer system of claim 7 wherein, when said selection unit selects
one of said processors, said statistical unit updates at least said
utilization ranking for said one of said processors.
9. The computer system of claim 7 wherein said new processor initialization
unit assigns to said newly available processor an initial utilization
ranking that is a mean of said utilization rankings for other said
processors.
10. The computer system of claim 7 wherein said new processor
initialization unit assigns to said newly available processor an initial
utilization ranking that is a median of said utilization rankings for
other said processors.
11. A computer system comprising:
at least one processor, and a plurality of stored data objects and a
plurality of cached data objects available to each of said at least one
processor; and
a selection unit for, when a stored data object is retrieved by one of said
at least one processor and is to become one of said cached data objects,
selecting which other one of said cached data objects should be purged
from said cached data objects based on historical data regarding
utilization of said cached data objects, said selection unit comprising:
a statistical unit for maintaining utilization rankings for said cached
data objects based on historical data regarding utilization of said cached
data objects by said at least one processor, one of said cached data
objects having a minimum utilization ranking and another of said cached
data objects having a maximum utilization ranking, said statistical unit
identifying said one of said cached data objects having said minimum
utilization ranking, and
a new object initialization unit for assigning to a newly cached data
object, said newly cached data object not initially one of said cached
data objects, an initial utilization ranking between said minimum
utilization ranking and said maximum utilization ranking.
12. The computer system of claim 11 wherein, when one of said at least one
processor accesses one of said cached data objects, said statistical unit
updates at least said utilization ranking for said one of said cached data
objects.
13. The computer system of claim 11 wherein said new object initialization
unit assigns to said newly cached data object an initial utilization
ranking that is a mean of said utilization rankings for other said cached
data objects.
14. The computer system of claim 11 wherein said new processor
initialization unit assigns to said newly available processor an initial
utilization ranking that is a median of said utilization rankings for
other said cached data objects.
15. A computer system comprising:
a plurality of processors, said processors being connected for
communication one with another;
a plurality of resources other than processors, each of said resources
being located at one of said processors but being available to other said
processors; and
a selection unit for, when there is a candidate group of said processors at
one of which a respective one of said resources should be located based on
utilization of said respective one of said resources by respective ones of
said processors in said group, determining at least one of (a) which
processor in said group should be eliminated from said group when a new
candidate processor is added to said group, and (b) at which processor in
said group said one of said resources should be located, said selection
unit comprising:
a statistical unit for maintaining, for a respective one of said resources,
rankings of utilization of said one of said resources by each of said
processors in said group based on historical data regarding utilization of
said one of said resources by said processors in said group, utilization
of said one of said resources by one of said processors in said group
having a minimum utilization ranking and utilization of said one of said
resources by another of said processors in said group having a maximum
utilization ranking, said statistical unit identifying at least one of (a)
said one of said processors associated with said minimum utilization
ranking for elimination from said group, and (b) said one of said
processors associated with said maximum utilization ranking for location
thereat of said one of said resources, and
a new candidate initialization unit for assigning to utilization of said
one of said resources by a processor that is newly added to said group,
said newly added processor not initially in said group, an initial
utilization ranking between said minimum utilization ranking and said
maximum utilization ranking.
16. The computer system of claim 15 wherein, when one of said processors in
said group accesses one of said resources, said statistical unit updates
at least said utilization ranking for utilization of said one of said
resources by said one of said processors in said group.
17. The computer system of claim 15 wherein said new candidate
initialization unit assigns to utilization of said one of said resources
by said new candidate processor an initial utilization ranking that is a
mean of said utilization rankings for utilization of said one of said
resources by said processors in said group.
18. The computer system of claim 15 wherein said new candidate
initialization unit assigns to said new candidate processor an initial
utilization ranking that is a median of said utilization rankings for
utilization of said one of said resources by said processors in said
group.
19. For use in a computer system having a plurality of resources of
different types, at least some of said resources being in a group of
resources from which a resource is selected, said resources being
connected for communication one with another and utilization one by
another, a method for selecting among said resources of a particular type,
said method comprising:
when a particular type of resource is required to be selected, selecting
among said resources of said particular type based on historical data
regarding utilization of said resources, said selecting step comprising:
maintaining utilization rankings for said resources based on historical
data regarding utilization of said resources, wherein, for a particular
subset of said resources comprising a plurality of members of said
particular type, one of said members has a minimum utilization ranking and
another of said members has a maximum utilization ranking, and identifying
said one of said members having one of (a) said minimum utilization
ranking, and (b) said maximum utilization ranking,
determining at least one of (i) which resource should be eliminated from
said group, when a new resource is added to said group, and (ii) which
resource of said particular type should be selected, based on historical
data regarding utilization of said resources, and
assigning to a resource that is newly added to said group, said newly added
resource being of said particular type and not initially in said group, an
initial utilization ranking between said minimum utilization ranking for
said one of said types and said maximum utilization ranking for said one
of said type.
20. The method of claim 19 further comprising, when a particular resource
of one of said types is utilized, updating at least said utilization
ranking for said particular resource of said one of said types.
21. The method of claim 19 wherein said assigning comprises assigning to
said newly available resource an initial utilization ranking that is a
mean of said utilization rankings for said one of said types.
22. The method of claim 19 wherein said assigning comprises assigning to
said newly available resource an initial utilization ranking that is a
median of said utilization rankings for said one of said types.
23. The method of claim 19 wherein said one of said types is cached data
objects.
24. The method of claim 19 wherein said one of said types is processors.
25. For use in a computer system having a plurality of processors, each of
said processors capable of performing tasks to be carried out by said
computer system, a method for, when a particular task is to be carried
out, selecting a processor from among a group of at least some of said
processors, said method comprising:
maintaining utilization rankings for said processors in said group based on
historical data regarding utilization of said processors in said group,
one of said processors in said group having a minimum utilization ranking
and another of said processors in said group having a maximum utilization
ranking, and identifying said one of said processor in said group having
said minimum utilization ranking,
determining at least one of (i) which processor should be eliminated from
said group, when a new resource is added to said group, and (ii) which
processor of said group should be selected, based on historical data
regarding utilization of said resources, and
assigning to a processor that is newly added to said group, said newly
added processor not initially in said group, an initial utilization
ranking between said minimum utilization ranking and said maximum
utilization ranking.
26. The method of claim 25 further comprising, when one of said processors
in said group is selected, updating at least said utilization ranking for
said one of said processors in said group.
27. The method of claim 25 wherein said assigning comprises assigning to
said newly added processor in said group an initial utilization ranking
that is a mean of said utilization rankings for other said processors in
said group.
28. The method of claim 25 wherein said assigning comprises assigning to
said newly added processor in said group an initial utilization ranking
that is a median of said utilization rankings for other said processors in
said group.
29. For use in a computer system having at least one processor, and a
plurality of stored data objects and a plurality of cached data objects
available to each of said at least one processor, a method for, when a
stored data object is retrieved by one of said at least one processor and
is to become one of said cached data objects, selecting which other one of
said cached data objects should be purged from said cached data objects,
said method comprising:
maintaining utilization rankings for said cached data objects based on
historical data regarding utilization of said cached data objects by said
at least one processor, one of said cached data objects having a minimum
utilization ranking and another of said cached data objects having a
maximum utilization ranking; and identifying said one of said cached data
objects having said minimum utilization ranking,
determining which cached data object should be eliminated from said cached
data objects, when a stored data object is newly cached, and
assigning to a newly cached data object, said newly cached data object not
initially one of said cached data objects, an initial utilization ranking
between said minimum utilization ranking and said maximum utilization
ranking.
30. The method of claim 29 further comprising, when one of said at least
one processor accesses one of said cached data objects, updating at least
said utilization ranking for said one of said cached data objects.
31. The method of claim 29 wherein said assigning comprises assigning to
said newly cached data object an initial utilization ranking that is a
mean of said utilization rankings for other said cached data objects.
32. The method of claim 29 wherein said assigning comprises assigning to
said newly available processor an initial utilization ranking that is a
median of said utilization rankings for other said cached data objects.
33. For use in a computer system having a plurality of processors, said
processors being connected for communication one with another, a plurality
of resources other than processors, each of said resources being located
at one of said processors but being available to other said processors, a
method for, when there is a candidate group of said processors at one of
which a respective one of said resources should be located based on
utilization of said respective one of said resources by respective ones of
said processor in said group, determining at least one of (a) which
processor in said group should be eliminated from said group when a new
candidate processor enters said group, and (b) at which processor in said
group said one of said
resources should be located, said method comprising:
maintaining, for a respective one of said resources, rankings of
utilization of said one of said resources by said processors in said group
based on historical data regarding utilization of said one of said
resources by said processors in said group, utilization of said one of
said resources by one of said processors in said group having a minimum
utilization ranking and utilization of said one of said resources by
another of said processors in said group having a maximum utilization
ranking, and identifying (a) said one of said processors associated with
said minimum utilization ranking for elimination from said group, and (b)
said one of said processors associated with said maximum utilization
ranking for location thereat of said one of said resources,
determining at least one of (i) which processor should be eliminated from
said group, when a new processor enters said group, and (ii) at which
processor in said group said one of said resources should be located, and
assigning to utilization of said one of said resources by a processor that
is newly added to said group, said newly added processor not initially in
said group, an initial utilization ranking between said minimum
utilization ranking and said maximum utilization ranking.
34. The method of claim 33 further comprising, when one of said processors
in said group accesses one of said resources, updating at least said
utilization ranking for utilization of said one of said resources by said
one of said processors in said group.
35. The method of claim 33 wherein said assigning comprises assigning to
utilization of said one of said resources by said new candidate processor
an initial utilization ranking that is a mean of said utilization rankings
for utilization of said one of said resources by said processors in said
group.
36. The method of claim 33 wherein said assigning comprises assigning to
utilization of said one of said resources by said new candidate processor
an initial utilization ranking that is a median of said utilization
rankings for utilization of said one of said resources by said processors
in said group.
37. For use in a computer system having at least one group of resources of
the same type, a method for selecting at least one resource from the at
least one group, the method comprising:
maintaining a utilization ranking for each resource in the at least one
group,
determining at least one of (i) which resource should be eliminated from
the at least one group, when a new resource is added to the at least one
group, and (ii) which resource of the at least one group should be
selected, based on historical data regarding utilization of said
resources, and
assigning to a resource that is newly added to the at least one group, the
newly added resource being of the same type as the resources in the at
least one group and not initially in the at least one group, an initial
utilization ranking between the minimum utilization ranking for the at
least one group and the maximum utilization ranking for the at least one
group.
38. The method of claim 37, wherein the resources in the at least one group
are of processors type.
39. The method of claim 38, wherein the resources in the at least one group
are of cached data objects type.
40. For use in a computer system having at least one processor and at least
one group of cached data objects available to the at least one processor,
a method for selecting at least one cached data object for elimination
from the at least one group, the method comprising;
maintaining a utilization ranking for each cached data object in the at
least one group
determining which cached data object should be eliminated from the at least
one group, when a data object is newly cached and added to the at least
one group, and
assigning to a data object that is newly cached and added to the at least
one group, said newly cached and added data object not initially in the at
least one group, an initial utilization ranking between the minimum
utilization ranking for the at least one group and the maximum utilization
ranking for at least one group.
41. For use in a computer system having at least one group of processors
and a plurality of resources other than processors, each resource from the
plurality of resources other than processors being located at a processor
from the at least one group of processors, a method for selecting at which
processor a resource from the plurality of resources other than processors
should be located, the method comprising:
maintaining, for one resource from the plurality of resources other than
processors, a ranking of the utilization of the resource by each of the
processors in the at least one group,
determining at least one of (i) which processor should be eliminated from
the at least one group, when a new processor is added to the at least one
group, and (ii) at which processor in the at least one group the one
resource from the plurality of resources other than processors should be
located, and
assigning to a processor that is newly added to the at least one group, the
newly added processor not initially in the at least one group, an initial
utilization ranking between the minimum ranking of utilization of the
resource by any processor in the at least one group and the maximum
ranking of utilization of the resource by any processor in the at least
one group.
Description
BACKGROUND OF THE INVENTION
This invention relates to computer systems having a plurality of resources
which are utilized to varying degrees. More particularly, this invention
relates to such a computer system in which the selection of resources
depends on historical loading, and which provides a mechanism for handling
a resource that has no history, either because the resource is new or
because maintaining the history for all resources is too difficult or
expensive.
Computer systems or networks typically have multiple resources (processors,
data objects, memory devices, communications gateways, output devices,
etc.). In many cases, the system must select a resource from a group of
resources of a particular type. For example, a central processing unit
("CPU") in a multiprocessor system must be chosen to perform a task, a
storage server in a computer network must be chosen to host a data object,
a data item must be chosen to be evicted from a cache memory, etc. The
goal in such cases is to make a selection that will result in the fastest
or most efficient operation of the system or network. Frequently, such
selections are made based on utilization statistics. Thus, for whatever
type of resource is required, utilization statistics are examined and,
most commonly, that resource of the required type, that in the recent past
has had a utilization metric (e.g., rate or count) that is, depending on
the nature of the task, either the lowest or highest utilization metric of
all resources of that type, is selected. However, such a selection
technique can lead to anomalous or unintended results in certain
situations, particularly when a new resource, for which no usage
statistics are available, is added to the group of resources.
For example, in the case of a computer system having multiple processors,
the known selection technique for assignment of tasks to processors would
result in the selection of the processor that is least busy. However, if a
new processor is added to the system (or an existing processor that was
offline returns to active status), that processor will, by definition,
have the lowest utilization metric of any processor in the system.
Therefore, for a time, all new tasks will be assigned to that processor.
Because there is a lag in updating the utilization statistics, the new
processor will, at least temporarily, be swamped. Ultimately, the
statistics will reflect the high utilization, tasks will no longer be
assigned to that processor, and the system will stabilize. In the
meantime, however, system performance may be degraded.
As another example, in a computer system having a large number of
processors and a large number of other resources, it may be desirable to
locate each particular other resource at the same location as the
processor that most frequently uses it (i.e., to "co-locate" the other
resource with the processor), to reduce communication traffic and delay in
the system. To determine which of the various processors that requests a
particular other resource is the one that requests that resource most
frequently, it may be impractical, particularly in a very large system, to
keep statistics on every processor's utilization of every other resource.
Therefore, instead, it is known to keep statistics on a group of
processors that are the "best candidates" for co-location of a particular
other resource thereat. When a new processor requests the other resource,
it is a potential candidate and so is added to the group, replacing the
candidate with the lowest number of "hits." However, the new processor is
now the candidate with the lowest number of hits. Therefore, if a second
new processor requests the same other resource, it will replace the first
new processor in the list of candidates for co-location. One can envision
a situation where two new processors become the most frequent users of a
particular resource, but never get established on the list candidates for
co-location because each continually replaces the other on the list.
In another example, a computer system may have a cache capable of holding a
certain number of data objects that is less than the total number of data
objects that may need to be cached. The goal would be to cache those
objects most frequently used. The first time some new object is used, it
is not known whether it will become frequently used or not. Therefore, it
is typically inserted into the cache, and its utilization is monitored.
However, the utilization metric is "1" (i.e., one use) the first time the
object is placed in the cache, which by definition is the lowest
utilization metric for any object in the cache (except perhaps for other
objects with the same utilization metric). If a second new object arrives,
it will be inserted in the cache and, if the cache is full, the object
with the lowest utilization metric will be evicted from the cache.
Normally, that would be the previous "new" object. Even if the previous
new object is extremely popular, it may never get a chance to be
established in the cache, or it may not get established until much later
than it should, because it will be caught in a cycle of being inserted in
the cache and then evicted in favor of other new objects.
It is clear, then, that selection mechanisms that rely on historical
utilization data may not handle well the situation where a new resource,
for which there are no historical data, is added.
It would be desirable to be able to provide a computer system or network in
which selection of resources depends on historical loading, but which
provides a mechanism for handling a resource that has no history, either
because it is new or because its history has not been maintained.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a computer system or network
in which selection of resources depends on historical loading, but which
provides a mechanism for handling a resource that has no history, either
because it is new or because its history has not been maintained.
In accordance with the present invention, there is provided a computer
system having a plurality of resources of different types. At least some
of the resources are grouped in a group of resources from which a resource
is selected. The resources are connected for communication one with
another and utilization one by another. When a particular type of resource
is required to be selected, a selection unit selects among the resources
of that particular type. The selection unit includes a statistical unit
for maintaining utilization rankings for the resources based on historical
data regarding utilization of the resources. For a particular subset of
the resources comprising a plurality of members of the particular type,
one of the members has a minimum utilization ranking and another of the
members has a maximum utilization ranking. The statistical unit identifies
that one of the members having either the minimum utilization ranking, or
the maximum utilization ranking, as may be appropriate. A new resource
initialization unit assigns, to a resource that is newly added to the
group and is of one of the types, an initial utilization ranking that is
between the minimum utilization ranking for that one of the types and the
maximum utilization ranking for that one of the types.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects and advantages of the invention will be
apparent upon consideration of the following detailed description, taken
in conjunction with the accompanying drawings, in which like reference
characters refer to like parts throughout, and in which:
FIG. 1 is a schematic diagram of a preferred embodiment of a computer
system according to the present invention;
FIG. 2 is a schematic diagram of a preferred embodiment of a server in the
computer system of FIG. 1; and
FIG. 3 is a flow diagram showing a preferred embodiment of the method of
operation of the computer system of FIGS. 1 and 2.
DETAILED DESCRIPTION OF THE INVENTION
The present invention alleviates some of the difficulties described above
in systems in which resources are selected based on historical utilization
rates, and particularly the difficulties in such systems, where the
resource with the lowest or highest utilization metric is selected to
perform a task or to be evicted from whatever group of resources is being
considered, that arise when a new resource is added to the group. The
solution of the present invention is to assign to the new resource an
arbitrary initial utilization metric that does not accurately reflect the
past utilization of the resource, either because the resource is brand new
and has never been used (or has been used only once), or because no past
utilization data are available.
It should be noted that while the utilization statistics have been referred
to as utilization "rates," the utilization rate frequently, and even
preferably, may be as simple as a count of the number of times the
resource has been used (or has used another resource), possibly limited to
a particular time period. Such a count would preferably be incremented
each time the resource was used (or used another resource).
The arbitrary initial utilization metric assigned in accordance with the
invention preferably places the resource approximately in the middle of
the group under consideration from a utilization standpoint. Preferably,
the initial utilization metric assigned to the new resource is either the
median or the mean of the utilization metrics of the other members of the
group. Assigning an initial utilization metric substantially equal to the
mean utilization metric for the group is particularly preferred because as
each individual utilization metric is updated, it is relatively easy to
modify the mean incrementally in constant time, by adding to the previous
mean value the quotient of (a) the change in the utilization metric of the
group member whose utilization metric has changed, and (b) the number of
members of the group. Determining the median, while not difficult, could
not be done incrementally and would require more computation.
By assigning to the new resource an initial arbitrary utilization metric
that is neither the lowest in the group nor the highest in the group, one
assures that the new resource initially will not be selected by a
selection process that requires either a minimum utilization metric (such
as those described above), or a maximum utilization metric (such as a
process for moving data resources to processors that most frequently use
them).
Although initially, in an application where the group member with the
minimum utilization is sought to perform a task, group members with higher
utilization metrics will be chosen when the new resource actually has the
minimum utilization metric, eventually, as the other members of the group
are selected and the new resource is not, their utilization metrics will
rise while that of the new resource remains constant, until it becomes the
minimum and is selected. However, no one resource will retain the minimum
utilization metric long enough to be overwhelmed, as would be the case
where the new resource is assigned its actual utilization metric as its
initial metric.
Similarly, in an application where the group member with the maximum
utilization is sought, although the new resource may prove extremely
popular, it will take some time before the arbitrary initial utilization
metric increases to reflect the actual utilization of the resource. During
that time other resources with apparently higher utilization metrics will
be selected, but eventually the utilization metric of the new resource
will become the highest and it will be selected. More importantly, the new
resource will not be eliminated from the group, and therefore from
consideration altogether, before it is given the opportunity for its low
initial utilization metric to be replaced by a higher utilization metric
that reflects its popularity and keeps it in the competition against other
resources.
The invention will now be described with reference to FIGS. 1-3.
A computer system of the type with which the present invention can be used
is shown in FIG. 1. As shown in this example, computer system 10 is a
local- or wide-area network, although it could also be a parallel, or
massively-parallel, processing system (not shown). System 10 has a
plurality of stations 11, and a plurality of resources including disk
drives and other storage systems 12, modems and other communications
facilities 13, as well as other types of resources (not shown).
Substantially all of the resources are generally available to each of
stations 11. Operations of system 10 are under the supervision of one or
more servers 14. As seen in FIG. 2, server 14 preferably has a processor
20 and directories 21 identifying the locations of various resources and
their association with particular stations 11 or with other resources.
Server 14 preferably has its own storage 22 which may be internal to
server 14 or in an external unit 12.
As discussed above, one or more of storage units 12 may hold cache memory
for various processes. When a data item not currently in the cache is
requested, it is added to the cache. Of the items already in the cache,
the item whose metric (e.g., rate or count) has the minimum value is
evicted from the cache. In accordance with the present invention, the
metric for the new member is preferably assigned to be the average of the
metric of the pre-existing members of the population, to prevent it from
automatically being evicted the first time room is required in the cache,
before it has an opportunity to establish its popularity.
Similarly as described above, various data objects can be placed on any of
the storage units 12 which are shown in physical proximity with particular
ones of stations 11. Physical location of resources 12 relative to the
stations 11 that use them may have significance if system 10 is large
enough, because then the added traffic on system 10 involved in routing
data to and from resources 12, and the associated time delays, may prove
unacceptable. It may therefore be desirable to locate a particular data
object on a storage unit 12 near one of stations 11 that uses it
frequently. When the number of candidate stations 11 for "co-location" of
the particular data object is high, it is impractical to maintain the
utilization statistics for each candidate and each data object. Instead,
the statistics can be kept on a small group of likely candidates for each
data object.
When a new candidate, not currently in the group, requests the data object,
it is added to the group, replacing an existing member with the lowest
metric (e.g., rate or count) for utilization of the data object. To
prevent that existing member from being the newest candidate, which has
not yet had a chance to show its true level of usage of the data object,
the metric for the new candidate is preferably assigned to be the average
of the metric of the pre-existing members of the candidate population.
When the time comes to select a member of the candidate group with which
to co-locate the data object, the member of the candidate group with the
highest utilization metric for this object is selected. The member that is
selected will have survived to achieve the highest utilization metric,
without being evicted from the candidate group, because at the time it was
added to the group it was assigned an arbitrary utilization metric in
accordance with the invention. At the same time, while other, newer
members of the candidate group are assigned arbitrary initial utilization
metrics higher than their actual utilization, those arbitrary initial
utilization metrics are not so high as to cause those other, newer
candidates to be inappropriately selected for co-location.
Similarly, if system 10 is a parallel processing system, and server 14
needs to assign a task to one of stations 11, it preferably would assign
the task to the station 11 with the lowest utilization metric (e.g., rate
or count). Again, to prevent the most recently added station 11 from being
assigned the most work, to the point that its performance, and that of the
whole system 10, degrades, its initial utilization metric preferably is
set to the average of the utilization metrics of the other stations 11.
FIG. 3 is a flow diagram of a portion of the system operating software for
assigning the metric to the new member of the population, the remainder of
the selection process being conventional. Process 30 starts at 31 where
the new member arrives. The process proceeds to test 32 to determine
whether there is room in the population for the new member, or if there is
not room and eviction of one of the older members is required. If there is
no room, then the process proceeds to step 33 where the member whose
metric for the particular characteristic being measured is lowest is
evicted to make room for the new member. After step 33, or if at test 32
there is room for a new member without eviction of an old member, the
process proceeds to step 34 where the mean metric for the preexisting
members of the population is calculated, and assigned arbitrarily to the
new member as its value for the metric, and the process ends.
Although in the process as described, the mean is used, the median could
also be used. However, in order to update the median whenever the metric
for one member changes (e.g., when the member is accessed by another
component of the system), a new calculation of the median would be
required. On the other hand, as stated above, if the mean is used, it can
be updated incrementally as any member is accessed by the system, simply
by adding to the old mean quotient of (1) the change in the value of the
metric for the accessed member, and (2) the number of members in the
population being examined.
Thus it is seen that a computer system or network in which selection of
resources depends on historical loading, but which provides a mechanism
for handling a resource that has no history, either because it is new or
because its history has not been maintained, has been provided. One
skilled in the art will appreciate that the present invention can be
practice by other than the described embodiments, which are presented for
purposes of illustration and not of limitation, and the present invention
is limited only by the claims which follow.
Top